Our railways aren't great. In fact, some of them are terrible. (As it happens, GWR, which is my usual company, isn't bad, but I know that elsewhere things are dire.) So, not unreasonably, many people feel we need to do something different, and the slogan is often 'Bring Back BR.' The only trouble is, British Rail was terrible too.
I had the relatively unusual experience of working for a company that was a nationalised industry when I joined it and which was then privatised after a few years. I have to say, it became a far better company as a result, both as a place to work and in the service it gave to its customers. (These days it's not doing so well, but then it's no longer really a British company.)
The reason, I'd suggest that the BA privatisation worked where BR one didn't is quite simple - we had real competition. Without that, privatisation is a joke. It can bring benefits, but unless tightly regulated it can leave us in the kind of mess the railways now are. Privatisation works when the customer has a choice. But if I want to travel, say, from Swindon to London it's GWR or nothing. It would be complicated to manage, I admit (but then so is airspace), but I honestly think the two keys to improving rail travel are:
a) Competition on major routes. At the very least all the really big routes should have a minimum of two companies operating on them. That means there's choice, and that means companies have to perform to keep your business. Of course there are real difficulties because of the limited amount of permanent way available, but airlines have to manage with very limited airport slots.
b) Some serious subsidy. This seems odd if we're talking about private companies - but environmentally speaking it doesn't make any sense for it to be cheaper to fly to places in Europe than it is to travel by train. Most other European countries manage their subsidies more effectively than we do. Again, it would not be simple, and EU regulations may well prevent it at the moment, but then...
I'm not doing a Donald Trump here. I realise this isn't going to be easy. Getting railways right is phenomenally difficult. But renationalising the railways isn't the answer.